Some processes performed on inorganic materials require the material to be in a molten state. Many such materials have a rather high melting point and are also corrosive in their molten form. One material which has particularly favorable qualities for use as a crucible for such materials is carbon, usually in the form of one of a number of different grades of graphite. Graphite has a theoretical density of 2.25 g/cc (grams per cubic centimeter) and a porosity between 10% and 25% of this theoretical density, depending upon the method by which it is manufactured. The porosity is a combination of closed and open porosity. Graphite can withstand very high temperatures and is unreactive with most of the inorganic materials for which processing in a carbon crucible would be contemplated. However, some inorganic materials which would otherwise be candidates for processing in a carbon crucible present special problems. Some leak through open pores in the graphite in their molten state and/or adhere to the graphite upon cooling to their solid state, so that they cannot be readily removed from the crucible without damage to either the inroganic material or the crucible. Alkali metal halide materials are in this latter category. For this reason, such materials are presently processed in their molten state only in precious metal crucibles, which are very costly.
Highly pure crystals of alkali metal halide material are useful as optical elements and as scintillation crystals in various types of radiation detectors. They may be sodium iodide (NaI), cesium iodide (CsI), or other compounds of this family. Sodium iodide doped with a very low concentration of thallium is of particular interest because it is widely used as a scintillation material for detecting x-rays for medical and other applications. The crystals are typically grown in a vertical Stockbarger furnace.
For growing sodium iodide in a vertical Stockbarger furnace, the raw material, which either already contains or has later added to it any desired dopants for establishing scintillation characteristics, is loaded in granular form as a charge into a crucible of platinum or other noble metal of the platinum series, platinum-rhodium alloy, platinum-iridium alloy, rhodium, or iridium. A noble metal is required in order to prevent contamination of the crystal with a more reactive material and also to withstand the highly corrosive effects of the alkali halide in its molten state.
The loaded crucible is placed into the upper portion of a Stockbarger furnace in a nonreactive gas. The furnace is then partially-evacuated and heated, so that all the material of the charge in the crucible becomes a melt. The crucible is then very slowly moved downward inside the furnace under controlled temperature conditions to move the freezing zone axially upwards inside the crucible while crystallization takes place, starting at the bottom of the crucible and moving upwards until all the melt has been crystallized to form a boule of monocrystalline material. By "monocrystalline" is meant that the boule does not consist entirely of microcrystalline material, but may consist of a plurality of crystals which are relatively large, much larger than microcrystals. Ideally, there is only a single crystal which makes up the boule. A plurality of crystals results in grain boundaries within the boule, which are undesirable discontinuities in the crystallinity and tend to trap impurities.
There are several serious drawbacks to the above-described present method of growing alkali halide crystals. The first is that the noble metal crucibles are very costly, costing on the order of hundreds of thousands of dollars each for making large crystals. They are generally made thin to accommodate differences in thermal expansion. Therefore, they also are made round to provide sufficient strength. This limits the crystal geometry to a round one and results in alkali halide material waste when rectilinear crystals are made from the crystal. Finally, and of particular significance, is that a meltout procedure is required to remove the boule from the crucible. The cooled boule adheres tenaciously to the surface of the crucible. In order to remove the boule without destroying the crucible and the boule in the process, it is necessary to heat the crucible until the boule surface region in contact with it remelts to permit its separation from the crucible. This remelting procedure is both time consuming, costly, and hazardous. The remelting takes place at temperatures above 800.degree. C. and is accompanied by the outgassing of toxic fumes from the boule which necessitates the provision of costly ventilation and other safety equipment.
Fused silica or fused quartz can be used as a crucible material, but these present problems similar to those for platinum in that the crystal must be melted out hot before cooling to room temperature. They are also fragile crucible materials.
There is therefore a need for an improved process for growing alkali halide crystals. One approach has been to use a crucible of carbon, such as graphite. Graphite has been used as a crucible material for growing other materials, such as alkali earth halide, calcium fluoride, and barium fluoride. It has the desirable properties of being very resistant to corrosion by these inorganic crystal materials, being able to withstand the high temperature needed to melt the crystal material, and resulting in little contamination. Unfortunately however, graphite is porous. When it is used as a crucible material for alkali metal halide crystal growth, the melt leaks into and through the crucible, thus making such a crucible unsuitable for alkali metal halide crystal growth. In addition, alkali metal halide and other materials tend to adhere to the surface of the graphite upon cooling, thereby preventing their ready removal from the crucible without damage to either the boule or the crucible. Thus far, no feasible alternatives to noble metal crucibles have been found for commercially growing alkali metal halide crystals.